Elsevier

Synthetic Metals

Volume 157, Issue 24, December 2007, Pages 1029-1033
Synthetic Metals

Measurement of carrier transport and injection in metal-free tetraphenylporphyrin

https://doi.org/10.1016/j.synthmet.2007.10.014Get rights and content

Abstract

Carrier transport and charge injection are studied in metal-semiconductor structures employing metal-free tetraphenylporphyrin (H2-TPP) as the organic semiconductor. H2-TPP is deposited on an indium tin oxide (ITO) substrate by means of an apparatus for molecular beam depositions, and aluminum is employed as the top electrode. The ITO/H2-TPP/Al structures thus obtained are investigated by a large-signal capacitance-voltage method, and transport and charge injection are simultaneously measured. At low electrical fields a space-charge limited transport is found, and the device behavior is symmetrical, notwithstanding the different energy barriers at the ITO and Al electrodes. At higher electrical fields the transport regime becomes Schottky-barrier limited, with the asymmetry expected from the energy difference between the two contact metals. The charge injected into the device behaves symmetrically at low fields, and shows a peak at the transition voltages between the space-charge and the Schottky regime, both in the positive and negative bias direction.

Introduction

In recent years, the family of porphyrin compounds has been largely investigated [1], because their close relation to chlorophyll suggests the possibility of important applications in photovoltaic cells [2], [3] and electroluminescent devices [4]. Among the different molecules of this family, metal-free tetra-phenyl-porphyrin (H2-TPP) appears promising for the development of organic electronic devices, thanks to its extended system of π-electrons. Moreover, the two hydrogen atoms bonded to the porphyrin ring are easily substituted for by a metal atom (typically a transition metal, like Zn, Mn, Cr, Al and others), changing the optical and electronic properties of interest [5].

It is, therefore, worthwhile to study the electrical properties of H2-TPP, especially those governing the charge transport in the bulk and those controlling the interface properties at the device contacts. The most important bulk property, the carrier mobility, is generally found to be very low in H2-TPP [6], of the order of 10−9 cm2 V−1 s−1. However, the carrier transport in organic semiconductors is usually modeled by the disorder formalism [7], where the low mobility derives from hopping among positionally and energetically disordered sites. It is, therefore, to be expected that the mobility would improve substantially in H2-TPP with a high degree of supramolecular order; this order could be attained (at least partially) by molecular beam deposition in high vacuum of the H2-TPP film.

The difference between the HOMO or LUMO of H2-TPP and the work function of some common metals, like Al or Au, [8] makes it difficult to obtain good ohmic contacts and usually leads to the formation of a Schottky-barrier. An important scientific and technological issue is the band alignment at the H2-TPP/metal interface; in the absence of interface states, the energy barrier between H2-TPP and metal should be given simply by the difference between their work functions. Some results [9] indeed confirm this result, while other experiments [10] suggest instead the presence of interface states, at least between H2-TPP and some metals.

In our study, we measured the carrier transport and injection in ITO/H2-TPP/Al structures by means of a large-signal capacitance-voltage method [11], [12], which permits to study the carrier transport and injection down to very low frequencies. The method will be briefly outlined here (for a more comprehensive description see [11], [12]). A triangular voltage waveform of frequency f and amplitude ±V0 is applied to a device, which is modeled with a conductance G and a capacitance C in parallel (both parameters are functions of the bias voltage V). The resulting current is the sum of two contributions:i(t)=G(V)V(t)+dQdt=G(V)V(t)+dQdVdVdt

In the resulting expression at right, the first term G(V)V(t) is due to the sample conductance and represents the conduction phenomena (ohmic, Schottky, space-charge limited, etc.). The second term, containing the time-derivative of the applied voltage, represents the charge exchanged between the device and the voltage source (electrostatic charge, interface states, injected charge, etc.). This second term will be called effective capacitance. Being the voltage waveform triangular, the time-derivative of the applied voltage is simply given by:dVdt=±4V0fwhere the positive and negative signs holds for the rising and falling part of the voltage, respectively. The conduction current ic(V) and the effective capacitance dQ/dV are given by [12]:ic(V)=12[i+(V)+i(V)]dQdV=i+(V)i(V)4V0fwhere i+ and i are the currents measured during the rising and the falling part of the voltage waveform, respectively.

Section snippets

Experimental

The experiments have been carried out on ITO/H2-TPP/Al devices, where the H2-TPP layer was sandwiched between an ITO (indium-tin oxide) and an Al-electrodes. H2-TPP has been evaporated onto a silica substrate, covered by an ITO layer having a thickness of 150 nm and a specific resistance of 50 Ω per square. Prior to evaporation the ITO substrate was cleaned in ultrasonic baths of acetone and methanol, rinsed in de-ionized water and dried in N2 flow. Evaporation was carried out in a molecular beam

Results and discussion

Fig. 1 shows the current density at low bias voltages (|Vbias|<3 V), measured at a frequency of 1 Hz for three devices with different thickness. Currents for positive and negative bias are shown separately in the two panels. The current shows a transition from a linear (ohmic) regime to a square-law suggesting a space-charge limited (SCL) conduction mechanism [15]:J=98εμV2L3where ɛ is the dielectric constant, μ is the carrier mobility and L is the thickness of the H2-TPP layer. In order to check

References (18)

  • K. Takahashi et al.

    Synth. Met.

    (2005)
  • A. Chowdhury et al.

    Synth. Met.

    (2001)
  • Y. Harima et al.

    Thin Solid Films

    (1997)
  • Y. Harima et al.

    Thin Solid Films

    (2000)
  • C. Calcavento et al.

    Synth. Met.

    (2003)
  • E. Pinotti et al.

    Synth. Met.

    (2003)
  • R. Tubino et al.

    Opt. Mater.

    (1998)
  • N.R. Armstrong

    J. Porphyrins Phthalocyanines

    (2000)
  • C.H.M. Marée et al.

    J. Appl. Phys.

    (1996)
There are more references available in the full text version of this article.
View full text